The manufacture of integrated circuits requires a method of forming patterns on wafers that is both accurate and precise to delineate areas for subsequent processing, such as dielectric deposition, ion implantation, and metallization. The process technology involved in the generation of these patterns is known as photolithography. A typical photolithography process sequence generally comprises the application of one or more uniform photoresist layers on the surface of a wafer substrate, drying and curing the deposited layers in a bake station, patterning the substrate by exposing the photoresist layer to electromagnetic radiation, and then developing the patterned photoresist layer.
The bake station in a photolithography tool is designed to heat wafers to process, cure, or activate a layer of material applied to the top surface of the wafer. The bake station comprises a bake plate, which is usually positioned below a wafer to heat the wafer backside without conducting heat directly to the back surface of the wafer. The heat is transferred to the wafer through an air gap using radiative and convective heat transfer. This configuration provides a way to heat the photoresist indirectly by transferring the heat through the air gap and substrate with minimal surface contact, which minimizes contamination due to physical contact to either the top or backside of the wafer.
However, the configuration of conventional bake stations may create two process control issues; temperature control of the hot plate and the control of heat transfer through the air gap. The bake plate temperature may be difficult to control during processing due to a rapid loss of energy when a cool wafer is placed in close proximity to the hot plate. Also, any variation in substrate flatness creates a non-uniform air gap spacing and may result in non-uniform heating of the wafer.
Further, conventional bake stations may also comprise a lid that prevents evolved solvents from escaping into the environment during processing while the photoresist layer is heated or cured. The lid may provide a cooler region where evolved solvents may condense, accumulate, and later fall back onto the topside of the wafer, thereby providing a source of contamination to the wafer.
What is needed, therefore, are improved apparatus and methods for heating a layer carried on a rotating substrate.